In a typical residential or commercial furnace, a fuel, for example natural gas, is combusted in a burner to heat air provided through an air intake vent pipe. Heat is extracted from the combustion gases through a heat exchanger, then the combustion gases are exhausted from an exhaust vent pipe. A byproduct of natural gas combustion is water vapor which, in lower efficiency furnaces, is typically exhausted with other combustion gases from the exhaust vent pipe.
In higher efficiency furnaces, the combustion gas can be routed through a primary heat exchanger and also a secondary heat exchanger to improve the extraction of heat to improve overall efficiency. The improved extraction of heat lowers the temperature of the exhaust gases and results in condensation of the water vapor into liquid water within the heat exchanger. This water is collected and temporarily stored in a condensate trap until it is disposed of, typically by draining the collected water from the condensate trap via gravity into a drain or onto the ground.
Gas furnaces are sometimes installed in an unheated attic or garage where the ambient temperature can drop below freezing. A lower efficiency furnace which exhausts the water vapor at higher temperatures can be used for these installations, as the moisture remains a vapor until after it exits the exhaust pipe. To use a high efficiency furnace in locations where freezing temperatures are a concern, freeze protection can be used to prevent freezing of the collected water within the condensate trap. Freeze protection can include an electrical heating tape which is wound around the condensate trap to prevent freezing. Electrical heating tape is typically configured for 120 volts, which can require additional safety precautions such as ground fault circuit interrupt (GFCI) breakers, special enclosures, or other measures. In some future systems, the use of electrical heat tape may not be possible because of small spacing around the condensate trap. Other freeze protection is described in U.S. Pat. No. 6,675,795, which is incorporated herein by reference in its entirety.